toolchain: add trailing _ to iterable section name

Add a trailing _ to the section iterable name. This is useful, for
example, to implement numeric sorting like this:

SORT(.z_device_LEVEL_?_)
SORT(.z_device_LEVEL_??_)

Without the trailing _ it would not be possible to use the ?? wildcard
without triggering into trigraphs issues, because linker-defs.h header
is included in C files as well.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>